This book argues that moral philosophy should be based on seven scientific principles of theory selection. It then argues that a new moral theory—Rightness as Fairness—satisfies those principles more successfully than existing theories. Chapter 1 explicates the seven principles of theory-selection, arguing that moral philosophy must conform them to be truth-apt. Chapter 2 argues those principles jointly support founding moral philosophy in known facts of empirical moral psychology: specifically, our capacities for mental time-travel and modal imagination. Chapter 2 then shows that these capacities present human decisionmakers with a problem of diachronic rationality that includes but generalizes beyond, L.A. Paul’s problem of transformative experience: a problem that I call “the problem of possible future selves.” Chapter 3 then argues that a new principle of rationality—the Categorical-Instrumental Imperative—is the only rational solution to this problem, as it requires our present and future selves to forge and uphold a recursive, bi-directional contract with each another given mutual recognition of the problem. Chapter 4 then shows that the Categorical-Instrumental Imperative has three identical formulations analogous but superior to Immanuel Kant’s various formulations of his ‘categorical imperative.’ Chapter 5 shows that these unified formulas jointly entail a particular test of moral principles: a Moral Original Position similar to John Rawls’ famous ‘original position’, but which avoids a variety of problems with Rawls' model. Chapter 6 then shows that the Moral Original Position generates Four Principles of Fairness, which can then be combined into a single principle of moral rightness: Rightness as Fairness. This new conception of rightness is shown to reconcile four dominant moral frameworks (deontology, consequentialism, virtue ethics, and contractualism), as well as entail a new method of moral decisionmaking for applied ethics: a method of “principled fair negotiation” according to which applied ethical issues cannot be wholly resolved through principled debate, but must instead be resolved by actual negotiation and compromise. This method is then argued to generate novel, nuanced analyses of a variety of applied moral issues, including trolley cases, torture, and the ethical treatment of nonhuman animals. Chapter 7 then shows that Rightness as Fairness reconciles three leading political frameworks—libertarianism, egalitarianism, and communitarianism—showing how all three embody legitimate moral ideals that can, and should, be fairly negotiated against each other to settle the scope, and nature, of domestic, international, and global justice on an ongoing, iterated basis. Finally, Chapter 8 argues that Rightness as Fairness satisfies all seven of the principles of theory-selected defended in Chapter 1 more successfully than rival theories.